Behavioural determinants of vector competence

Arthropod behaviours, especially those related to pathogen transmission, are thought to be genetically inherited (Lehane, 1991). Environmental factors such as temperature, relative humidity and precipitation as well as host cues like CO2 and heat release alter the biochemistry and physiology of the vector, initiating specific behavioural responses. Host preference and seeking, feeding, and oviposition behaviour are probably the most important behavioural determinants for vector competence and, due to their complexity, will be briefly discussed.

Host-seeking behaviour. The difficulty that host-seeking arthropods have in locating their next blood meal depends upon the closeness of their association with the host, their lifestyles (e.g. permanent vs temporary feeders) and mobility (e.g. flying vs crawling/jumping). Sutcliffe (1987) suggested that host-searching behaviour could be divided into three phases: appetitive searching, activation and orientation, and attraction. These behaviours are quite versatile and do not occur necessarily in a strict sequence. Lehane (1991) offers a more comprehensive discussion of these behaviours. Appetence initiates a series of behavioural responses that lead to host contact and successful blood feeding. Passive and active are two basic strategies used by arthropod vectors to find hosts; passive arthropods such as many ticks remain quiescent in their habitat and depend upon contact with the vertebrate animals that invade it, while active arthropods like biting flies or kissing bugs leave their resting environments and fly or walk in search of hosts. In some instances, the release of substances such as an aggregation-attachment pheromone by Amblyomma variegatum and Amblyomma hebraeum ticks triggers the movement of hungry ticks toward areas of the host already infested. Because these two species have been implicated in the transmission of several animal rickettsioses and one arbovirus (Sonenshine, 1993) this aggregation behaviour may increase the number of ticks that become infected and transmit.

Heat can act as a vector attractant. The louse (Pediculus humanus) vector of typhus rarely leaves the host but will do so if body temperature and humidity change drastically, such as following a high fever or death of the host (Kettle, 1983). The louse can then move as a far as 2 metres in search of a new host, using heat as a locating signal. This behaviour increases the chances of encountering a healthy host. In other vectors, such as Anopheles mosquitoes, host hyperthermia has no effect on vector feeding (Day & Edman, 1984).

Morphological characteristics of the host like body size can also determine the choice of host. For example, An. gambiae seems to prefer human adults over children (Port et al., 1980). Habitat distribution may also influence host selection. Host-seeking (questing) ticks near the ground are mostly exposed to small animals while those questing higher in the vegetation encounter larger animals. These differences are observed in the life stages of the American dog tick, Dermacentor variabilis, with immature ticks questing at ground level and parasitizing mice, rats and voles, versus adults questing higher in the vegetation on larger mammals including dogs and humans. These feeding patterns are highly relevant in the epidemiology of Rocky Mountain spotted fever (Sonenshine, 1993).

Host preference and feeding behaviour. One of the most important factors in determining vector-pathogen relationships is host preference and feeding behaviour.

Host choice may be determined by a combination of behavioural, physiological, morphological, ecological, geographical, temporal and genetic factors. Blood feeding by some vector taxa such as mosquitoes is restricted to females, while both males and females of other taxa (e.g. fleas) utilize blood as a source of nutrients for a variety of physiological activities, including reproduction (Lehane, 1991). The source of the blood meal or its quality may influence the course of pathogen infection in the vector. For example, tsetse flies feeding on goats or cattle develop a higher rate of infection with Trypanosoma vivax than those fed on mice (Maudlin et al., 1984).

Arthropod blood feeders can be divided into host-specific and opportunistic (Sonen-shine, 1993). Culiseta melanura, the enzootic vector of Eastern equine encephalitis virus (EEEV), feeds almost exclusively on passerine birds, the reservoir hosts (Scott & Weaver, 1989). Boophilus microplus, the tick vector of babesiosis and anaplasmosis of cattle, feeds almost exclusively on cattle; Ixodes scapularis and Ixodes pacificus, vectors of Lyme disease in North America, feed on a wide variety of vertebrates including large mammals, birds and reptiles. The mosquito Culex salinarius, a potential vector of WNV in North America (Sardelis et al., 2001), has a wide host range including birds, equines and canines. This species also takes multiple blood meals during one gono-trophic cycle if feeding is disturbed by defensive behaviours of the host (Cupp & Stokes, 1976). If infected with a pathogen, this behaviour increases the chance of transmission during a short period of time. On the other hand, the competence of the body louse Pediculus humanus humanus to transmit epidemic typhus and relapsing fever (Kettle, 1983) relies on its tendency to feed exclusively on humans.

Seasonal variation in host feeding and host preference can enhance transmission of pathogens and initiate disease outbreaks. For instance, Culex nigripalpus and Cx. tarsalis are vectors of St. Louis encephalitis virus (SLEV) during summer months in Florida and California, respectively. Both species show a marked seasonal change in their feeding patterns, switching from bird feeding in the winter and spring to mammal feeding in the summer when arbovirus transmission occurs (Edman, 1974; Edman & Taylor, 1968; Tempelis & Washino, 1967). These changes in feeding patterns may affect SLEV epidemiology. It has been suggested that the retreat of Plasmodium malariae and P. falciparum from Europe during the late 19th century was caused by a switch in host preference of several anopheline vectors from humans to domestic animals. Changes in animal husbandry, agricultural practices, housing and a decreased human birth rate may have reduced the number of human hosts available, contributing to reductions in transmission of malaria (Lehane, 1991).

Biological differences in host choice among species complexes have been well documented. An. gambiae sensu lato is composed of six sibling species in Africa.

Across its geographical range, one sibling species, Anopheles arabiensis, exhibits highly variable host-seeking (from exophilic to endophilic) and feeding (from anthropophilic to zoophilic) behaviours, compared to An. gambiae sensu stricto, which is primarily an endophilic species biting man. The sporozooite infection rates of An. arabiensis are about 1/15th those in An. gambiae, a highly efficient malaria vector. The higher sporozooite rates in An. gambiae are not a reflection of higher susceptibility to malaria parasites but arise because An. gambiae s.s. lives longer and feeds more often on humans (White, 1974).

Temporary blood feeders (e.g. flies) tend to take relatively large blood meals, thus limiting the number of host contacts and exploiting a blood source that may be available only temporarily. Blood meals taken by tsetse flies are two to three times the mass of their bodies, impairing manoeuvrability during flight from the host and decreasing flying speed significantly (Glasgow, 1961). Since these exothermic flies require a body temperature of 32 °C to enhance take-off, and feed around dusk and dawn, a time without optimum lift temperatures, they have adapted to avoid host defensive responses and escape predators. Immediately after the blood meal, the fly raises its thoracic temperatures towards the optimum by 'buzzing', producing the characteristic sound after which the tsetse is named. By increasing its thoracic temperatures, the fly maximizes its lift and flight speed, allowing it to take a large blood meal and depart from the host (Howe & Lehane, 1986). This behaviour ensures the success of pathogen infection in the vector and transmission to a new host. It may be more disadvantageous for permanent blood feeders closely associated to their hosts to take large blood meals. Many ectoparasites, such as anopluran lice, feed more frequently (every few hours) and take meals that are only 20-30 % of their body mass (Murray & Nicholls, 1965), ensuring the survival of the vector and increasing the chances of further pathogen transmission.

Mechanical transmission of pathogens may be facilitated by the feeding frequency of vectors. Arthropods that take a succession of partial blood meals from several vertebrate hosts are probably the most efficient mechanical vectors. Large insects, such as tabanid flies, are often disturbed by their vertebrate host before they complete a full blood meal. Tabanus spp. feed on large mammals usually found in herds, are excellent fliers, and possess sponging mouthparts. Because their painful bite triggers defensive behaviours in the host, a full blood meal is usually accomplished by additional visits to other vertebrate hosts. Moreover, the large amount of blood taken by these vectors increases the chances of pathogen transmission (Soulsby, 1982).

Oviposition behaviour. Environmental modification as a result of human incursions for agriculture or urbanization can affect the epidemiology of vector-borne diseases.

Human habitation and urbanization of enzootic ecosystems has triggered closer associations between human hosts and vectors. Most vector-borne disease transmission is the result of a close relationship between the vector and the affected host or a fortuitous encounter. Because many vectors travel short distances or wait passively during host seeking, the selection of an oviposition site can aid in bringing the vector and the host closer, thus enhancing transmission. Louse-borne typhus epidemics in humans, for example, are associated with overcrowded, unsanitary conditions, especially during times of war, famine and natural disasters (Kettle, 1983). The vector, P. humanus, lays its eggs in the thousands in clothing; this microenvironment, along with the conditions encountered during outbreaks, could facilitate the spread of the vector over large numbers of hosts in an explosive manner. Eggs of Rhodnius spp., Panstrongylus spp. and Triatoma spp., vectors of Chagas' disease in South America, are inserted in the rough surfaces, cracks and crevices of houses in rural and urban areas. Because their entire life cycle can occur inside human habitations using a variety of hosts, trypanosome transmission rates in humans are very high inside houses. In contrast, human cases of Chagas' disease occur in the US rarely in spite of the occurrence of the trypanosome, vector and reservoir, because the vectors rarely become domestic (Brenner & Stoka, 1987).

Mosquito oviposition behaviour, particularly site selection, has had a tremendous impact in the emergence and re-emergence of arboviral diseases. The introduction of YFV from Africa to the Neotropics probably occurred during the African slave trade (Chang et al, 1995). Water containers on-board ships crossing the Atlantic from Africa appear to be an ideal breeding site for introduction of Ae. aegypti into the Americas. Other mosquito vectors like Ae. albopictus, Ae. triseriatus and Culex quinquefasciatus have adapted to oviposit in peridomestic water containers, bringing arboviruses into closer contact with humans.

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